Diamond-like carbon (DLC) is a dense, partially sp³ bonded form of amorphous carbon prepared by ion beam or plasma deposition and widely used as a hard coating material. Its sp³ bonding arises from C⁺ ions penetrating surface layers, increasing local density. The formation of DLC can be viewed as a phase transition to a denser metastable phase. The atomic structure of DLC consists of a network of sp³ and sp² sites. The π states of sp² sites control electronic properties, while the connectivity of sp³ sites controls mechanical properties. DLC is a dense, metastable form of amorphous carbon (a-C) or hydrogenated amorphous carbon (a-C:H) with significant sp³ bonding. This sp³ bonding gives DLC valuable 'diamond-like' properties such as mechanical hardness, low friction, optical transparency, and chemical inertness. Although DLC films have poorer properties than diamond films, they have advantages like room temperature deposition, deposition on Fe or plastic substrates, and superior surface smoothness. DLC can be deposited by various methods including ion beam deposition, magnetron sputtering, ion sputtering, laser plasma deposition, plasma deposition, and ion plating. The common factor in these processes is deposition from a beam containing medium energy (10-500 eV) ions. Plasma deposition is the most popular method for producing a-C:H. The properties of DLC depend on the deposition process. The sp³ and H content is best displayed on a ternary phase diagram. The three corners correspond to diamond, graphite, and hydrocarbons. There is an excluded region at high H contents where molecular solids cannot form. The properties of DLC depend on the deposition process. The sp³ content and H content are most conveniently displayed on a ternary phase diagram. The three corners correspond to diamond, graphite, and hydrocarbons. There is an excluded region at high H contents where molecular solids cannot form. The familiar forms of non-crystalline carbon such as glassy carbon and evaporated a-C lie in the sp² corner. Sputtering methods produce hard but predominantly sp² bonded a-C. Ion-assisted sputtering onto well-cooled substrates can produce a highly sp³ bonded a-C. Laser plasma methods can produce highly sp³ bonded a-C films. A particularly useful form of ion beam deposition is filtered ion beam deposition, which allows deposition from a monochromatic, single species ion beam. The resulting a-C attains an sp³ content of up to 85% and is called highly tetrahedral a-C or 'ta-C'. The properties of ta-C depend strongly on the energy of the ion beam. The sp³ content, density, and hardness each pass through a maximum at an optimum ion energy of order 140 eV. Ta-C films also possess a very high intrinsic compressive stress arising from the deposition process, which is a key signatureDiamond-like carbon (DLC) is a dense, partially sp³ bonded form of amorphous carbon prepared by ion beam or plasma deposition and widely used as a hard coating material. Its sp³ bonding arises from C⁺ ions penetrating surface layers, increasing local density. The formation of DLC can be viewed as a phase transition to a denser metastable phase. The atomic structure of DLC consists of a network of sp³ and sp² sites. The π states of sp² sites control electronic properties, while the connectivity of sp³ sites controls mechanical properties. DLC is a dense, metastable form of amorphous carbon (a-C) or hydrogenated amorphous carbon (a-C:H) with significant sp³ bonding. This sp³ bonding gives DLC valuable 'diamond-like' properties such as mechanical hardness, low friction, optical transparency, and chemical inertness. Although DLC films have poorer properties than diamond films, they have advantages like room temperature deposition, deposition on Fe or plastic substrates, and superior surface smoothness. DLC can be deposited by various methods including ion beam deposition, magnetron sputtering, ion sputtering, laser plasma deposition, plasma deposition, and ion plating. The common factor in these processes is deposition from a beam containing medium energy (10-500 eV) ions. Plasma deposition is the most popular method for producing a-C:H. The properties of DLC depend on the deposition process. The sp³ and H content is best displayed on a ternary phase diagram. The three corners correspond to diamond, graphite, and hydrocarbons. There is an excluded region at high H contents where molecular solids cannot form. The properties of DLC depend on the deposition process. The sp³ content and H content are most conveniently displayed on a ternary phase diagram. The three corners correspond to diamond, graphite, and hydrocarbons. There is an excluded region at high H contents where molecular solids cannot form. The familiar forms of non-crystalline carbon such as glassy carbon and evaporated a-C lie in the sp² corner. Sputtering methods produce hard but predominantly sp² bonded a-C. Ion-assisted sputtering onto well-cooled substrates can produce a highly sp³ bonded a-C. Laser plasma methods can produce highly sp³ bonded a-C films. A particularly useful form of ion beam deposition is filtered ion beam deposition, which allows deposition from a monochromatic, single species ion beam. The resulting a-C attains an sp³ content of up to 85% and is called highly tetrahedral a-C or 'ta-C'. The properties of ta-C depend strongly on the energy of the ion beam. The sp³ content, density, and hardness each pass through a maximum at an optimum ion energy of order 140 eV. Ta-C films also possess a very high intrinsic compressive stress arising from the deposition process, which is a key signature